In X-ray tubes, the position of a filament with respect to a cathode cup must be controlled precisely to control the dimensions of the focal point of the X-rays. Conventionally, a filament is fixed to the cathode cup via an insulating member using steatite, ceramic, etc.
Lead wires are passed into a glass member of an envelope to supply electricity from outside of the X-ray tube to the filament. One or two lead wires are connected to the filament while the other lead wires are fixed to a cathode cover which supports the cathode cup.
During operation of the X-ray tube, the filament reaches a temperature of over 2000° C. Thus, cathode components in the proximity of the filament such as the cathode cup are heated by radiant heat from the filament. In most cases, the heat of the cathode cup transfers to the cathode cover and is released outside of the X-ray tube via the lead wires. Generally, a lead wire has a diameter of 1 to 2 mm and a length of 10 to 20 mm, and thus the cathode cup is nearly vacuum insulated. Therefore, the heat of the filament tends to increase the temperature of the cathode cup. From observation, the temperature of the cathode cup has been found to reach over 200° C. when the filament power is approximately 10 W.
The filament must be heated to a high temperature for the emission of thermoelectrons, while the other cathode components are desired to be kept at a low temperature to suppress the emission of gas inside the envelope. The desorption of the gas around the filament, which reaches an extremely high temperature, occurs in a short period; however, the desorption of the gas around the cathode components, which reach several hundreds of degrees centigrade, occurs slowly, and the emission of the gas is maintained over a long period such that the vacuum in the envelope is gradually degraded.